There has been long known a multi-point fuel injection system in which a fuel injector is provided for each of the cylinders, whereby fuel injection is effected to each of the individual cylinders independently from one another. The multi-point fuel injection system is effective for reducing the difference in the fuel quantity supplied to individual cylinders, to thereby enhance the fuel supply response characteristic of the fuel injection system in accelerating and decelerating operation modes by controlling with a high accuracy, the fuel quantity supplied to the individual cylinders.
However, since a plurality of expensive fuel injectors of high precision as well as an associated electronic control device are required corresponding to the number of the cylinders, the multi-point fuel injection system involves high manufacturing costs. Further, in order to attain a uniform fuel distribution to the individual cylinders, the fuel injectors have to be positioned properly at predetermined locations, which gives rise to the problem that the installing procedures as well as the maintenance and inspection become very complicated and troublesome.
As a measure for solving the above problems, there has been proposed a single-point fuel injection system in which a single fuel injector is disposed at the entry portion of an intake manifold of the engine and operated to inject fuel in an atomized state, whereby the resulting fuel air mixture is distributed to the individual cylinders in timing of the suction strokes thereof; (reference is made to U.S. Pat. No. 4,132,203 and the corresponding Japanese Laid-Open Patent Application No. 115411/1978).
The single-point fuel injection system is advantageous in that no more than one fuel injector or fuel injection valve device is required. However, because the fuel injection is effected for every suction stroke of all the individual cylinders, an expensive high speed type fuel injector which is capable of being operated reliably in response to the suction strokes of the individual cylinders is required, in order to supply the fuel air mixture having a uniform fuel air ratio over the entire operation range of the engine, which of course involves high costs to a disadvantage.
Further, in order to attain a uniform fuel distribution to the individual cylinders, the fuel injection initiating timing has to be set with a high accuracy, because otherwise, time lag may occur in the fuel supply to the cylinders, thereby making it difficult to attain a uniform fuel distribution.
For example, in the case of the single-point fuel injection system disclosed in U.S. Pat. No. 4,132,203, an arrangement is provided such that the fuel injection is initiated at a time point in advance of top dead center by 15.degree. in terms of the crank angle for every suction stroke of the individual cylinders in an effort to attain a uniform fuel distribution thereto. However, it should be noted that a high suction air flow rate is available only during the latter half of the suction stroke. At a time point is advance of top dead center by 15.degree. in terms of the crank angle, the air flow rate has been already reduced considerably.
Thus, the fuel injected at the time point at which the air flow rate is low cannot be supplied to the cylinders at a sufficient feeding rate and will undergo a delay, eventually resulting in non-uniform fuel distribution.
Further, the fuel injection is controlled by opening and closing an electromagnetic or solenoid valve. In this connection, it is noted that the maximum valve opening duration and hence the maximum fuel injection of the fuel injector which is adapted to effect the fuel injections for every suction stroke of the individual cylinders is previously determined in consideration of relation to other devices.
On the other hand, the operation characteristic of the electromagnetic valve is inherently such that the valve will not respond instantly to an actuation input signal, but will exhibit some delay in operation.
Consequently, in a high speed operation range of the engine in which the injection valve opening duration is set relatively short, the quantity of injected fuel will not be proportional to the injection valve opening time duration, but will become shorter, as compared with the engine operation in a low speed range in which the injection valve is opened for a relatively long time duration, and thus the influence of the electromagnetic valve to the proportional relationship between the valve opening duration of the fuel injector and the quantity of injected fuel can be neglected.
It is thus obvious that the prior art single-point fuel injection system such as disclosed in U.S. Pat. No. 4,132,203 in which fuel injection is effected for every suction stroke within the individual cylinders cannot avoid the disadvantage of non-uniform fuel distribution or a decreased fuel injection quantity in the high speed engine operation range due to the delay involved in the operation of the electromagnetic valve.